This invention relates generally to a spectacle line blind of the type used in pipelines to provide either full flow or no-flow operation between a source and a tank without modulated flow control, and more particularly, to a line blind which blocks entry of flow through the valve from the source to the tank while at the same time permitting venting of the tank. In the prior art, spectacle valves have been used on large pipe lines and ships to provide two conditions of operation. When the spectacle valve plate is in one position a straight-through unobstructed path is provided for gas flow through the valve. When the spectacle plate is rotated 180.degree. a solid panel is placed between the inlet and outlet of the valve entirely blocking the flow through the valve. Such a valve, when shut off and having proper seals, isolates the tank from the source effectively and simply and provides low flow resistance when opened. However, venting of the tank is not possible through the valve as constructed and other means must be used. For example, in the prior art vent valves have been designed for use in the supply line itself. The valve has an external cover which is removed when it is desired to provide venting. A wedge-shaped venting gate is then inserted in the valve and a cover is placed over the opening through which the venting gate is inserted. The venting gate includes ports which provide access to the tank, and the valve cover has vent holes such that vapor is vented from the tank. Another venting method using, for example, a plug in the supply pipe during venting is considered unsafe when venting cargo tanks because the plug may inadvertently be left in place when all other tank openings are closed causing overpressurization of the tank during loading. The wedge valve described above of the prior art requires the physical removal and insertion of the wedge-shaped venting gate and this consumes time and labor and complicates the venting procedure.
What is needed is a simple line blind which also permits venting.